Need Help: ZD28 Temperature Fluctuating/Red Lining at Start Up

BrianF8

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Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
I'm a new ZD28 owner (2003 72" prodeck with 525 hours) and trying to make sure everything is operating normally on my unit. I notice at start-up that my temperature sensor oscillates from cold to hot about 5 times before settling in at a middle temperature. It sounds like this is normal as the thermostat is helping to get the engine up to temp, but what concerns me is that it will red line for a good 5 second before rapidly dropping back down to low temp. Is this typical...or should I replace my thermostat?

I drained the coolant and flushed the system last night because there was some brown/gray sludge at the drain cock and having no service history on the mower I figured it was a good idea. For all I know, the prior owner may have mix coolant types or the sludge was a remnant from a previously blown head gasket. The thermostat physically looked OK and after refilling with a 50/50 distilled water/green coolant mix the temperature oscillations were still present. There's a bit of white smoke at start-up but gone prior to getting up to temperature.

I'm new to diesel equipment but learning rapidly so any advice is greatly appreciated.
 

BrianF8

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Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
Yes - I will be picking up a new thermostat and gasket immediately. I'm curious though...is it normal for the temperature to oscillate then settle down? I certainly don't see anything like that happening on other engines, but I'm not very familiar with diesel operation yet.
 

ShaunRH

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L3200
May 14, 2014
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Atascadero, CA
Yes - I will be picking up a new thermostat and gasket immediately. I'm curious though...is it normal for the temperature to oscillate then settle down? I certainly don't see anything like that happening on other engines, but I'm not very familiar with diesel operation yet.
You could have an electrical problem with the sender if it's an electric type, sounds like it could be a worn or frayed wire touching the block or a power line of some kind.

If it's an old analog thermal lead type, those shouldn't bounce at all, they tend to either work or not...

If it's some other type, I've never run across one before.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
I was also thinking electrical problem. I will trace the wiring to double check, but since the needle is stable after warming up I think it would maybe be a late opening thermostat which would explain the quick temp drop (as coolant runs past the sensor). My temp oscillations are only at start-up. Right now it reminds me of a very rigid control loop that is overshooting...again why I'm likely pointing the finger at the thermostat.

Appreciate the pointers...please keep them coming if you think I'm missing something.
 

Stubbyie

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Jul 1, 2010
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Midcontinent
I like your idea of a control loop that's overreacting without proper damping, but I think you'll find there's no logic aspect to the sensor: it's just a resistor. This assumes you have an electrical and not mechanical type.

My bet is your sensor is trying to go bad.

The probability of a thermostat 'stuttering' only on startup as you describe is low I think. They'll usually fail completely closed or open. A new thermostat would be an easy and relatively inexpensive endeavor to bring the machine back toward new[er] condition. Be certain you install the correct temperature new thermostat. Try using soft-set aviation-type Form-a-Gasket in addition to the basic 'paper' gasket.

Please post back your continuing experiences so we may all learn.
 

BrianF8

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Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
Thanks for the input Stubbyie. By the overdamped control loop, I'm referring to the operation of the thermostat not the resistive sensor that's moving the needle in my instrument cluster.

Once the new thermostat comes in I'll replace the old one. Just for the heck of it I was thinking of putting the old one in a pot of water on the stove and cranking up the temperature to see what it kicks open. Does anyone have temperature specs on the thermostat?

On teardown I'll try to get some pictures too.
 

Stubbyie

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Jul 1, 2010
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If you're buying from a dealer (or even an aftermarket like NAPA) the box or bag should have the thermostat temperature rating printed on it.

I don't know what the specs are, haven't had to change a Kubota in ever.

Track down a Shop Manual and should provide additional info.

Years ago I installed a new thermostat in a GM product for a neighbor. At their insistance (Lil' Widder Lady, 'LWL') I used a GM branded product from her dealer. Vehicle continued misbehaving and overheating. Finally pulled the 'new' termostat and parboiled it while measuring the water temp. Damn thing was bad straight new-from-the-box.

So the crux of this short story: you might consider 'cooking' your new thermostat before installing just to be certain it works as advertised.

Please post back your continuing experiences so we may all learn.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
Great idea about about cooking the new thermostat first. I'll go ahead and do that.

I got the new thermostat and gasket from the dealer this weekend but when I got home I noticed they stapled the two bags together and sent a staple right through the gasket...let's hope they have another one in stock for me.

I'll post the results of the fix this week.
 

kuboman

Member
Dec 6, 2009
725
4
16
Canada
Sounds like a bad sensor or gauge. Thermostats don't act like that. Good idea to change the thermostat though and also to check the new one. I have had more than one new thermostat not work properly.
 

Stubbyie

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Jul 1, 2010
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Can't say I've had a gasket complete with staple holes, but I have had fall into my hands several different gaskets with manufacturing imperfections---bolt holes incompletely punched, fuzzy edges, pressed-in-grooves and marks, miscut holes: the usual suspects.

I've gotten into the habit of using Form-a-Gasket #2 Softset Aviation Gasket Paste. It's a nasty brown alcohol-based material that seems to stay soft forever and, in later years, lets the now 'old gasket' come off intact. Expect handcleaner for your hands; the only cleaner that seems to work is methyl alcohol (methanol) which you don't want on your skin.

RTV has its place, but you won't later get the gasket loose intact and scraping can be a chore (you can get an RTV dissolver / cleaner in aerosol, but it's 'methyl-ethyl death' in a can) compared to simply wiping the mating surfaces with methanol.

Please post back your experiences so we may all learn.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
Thank you, very good advice. I'll go ahead and get some of that paste in addition to the new gasket.

I put both the old and new thermostats in a pot of water last night and gradually increased the temperature. The new, genuine Kubota thermostat opened sooner (at about 80C), quicker, and allows more fluid to pass once fully open. The thermostat I removed is aftermarket.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
Update: After installing the new thermostat and gasket, and refilling the radiator, the temperature still spikes at start-up and stays near the red line for a few seconds before dipping down and repeating several times. Before throwing in the towel and calling the dealer, my next thought is to replace the temperature sensor by the water pump. I pulled up the part diagram and it's called a "thermo-switch sensor". The radiator fan is connected directly to the engine output (along with the water pump), so I'm confused as to what it may switch? While browsing the parts diagram I found a thermo-sensor at the rear of the engine. I unplugged the leaded and did a resistance reading from the sensor to the body ground and saw it was 240 Ohms, and when shorting to ground the needle jumps to the red line on my temperature cluster. That looks like it's working properly.

Does anyone know what the "thermo-switch sensor" actually switches? Should I just replace it, see if that fixes it, and if not call the dealer? I notice white exhaust fumes for the first 15-20s of running too - not sure if that's related to the issue. That goes away before the temperature spikes start up.
 

kuboman

Member
Dec 6, 2009
725
4
16
Canada
Update: After installing the new thermostat and gasket, and refilling the radiator, the temperature still spikes at start-up and stays near the red line for a few seconds before dipping down and repeating several times. Before throwing in the towel and calling the dealer, my next thought is to replace the temperature sensor by the water pump. I pulled up the part diagram and it's called a "thermo-switch sensor". The radiator fan is connected directly to the engine output (along with the water pump), so I'm confused as to what it may switch? While browsing the parts diagram I found a thermo-sensor at the rear of the engine. I unplugged the leaded and did a resistance reading from the sensor to the body ground and saw it was 240 Ohms, and when shorting to ground the needle jumps to the red line on my temperature cluster. That looks like it's working properly.

Does anyone know what the "thermo-switch sensor" actually switches? Should I just replace it, see if that fixes it, and if not call the dealer? I notice white exhaust fumes for the first 15-20s of running too - not sure if that's related to the issue. That goes away before the temperature spikes start up.
One sensor will be for the gauge the other for the light.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
There is no over temperature light, just battery, oil pressure, and glow plugs. I unplugged the thermo-sensor switch and tied the wire to chassis ground and a loud horn went off - scared the crap out of me. I guess that explains what that switch does!
 

FTG-05

Active member

Equipment
L4330 w/FEL, RTV-XG850 and ZD326S
Jul 21, 2013
245
91
28
TN
I used to have a toyota 4runner that did the same thing.

When I bought a new thermostat, I made sure that it had a very small hole in it so that a small amount of hot fluid would flow through it even when cold. Very stable temps after installation. Hole was less 1/16" +/-.

Good luck.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
That certainly makes sense, I may try that once I verify the sensors are all OK.

One other thought - I've flushed my coolant system, but maybe I have some debris in the engine block that is preventing me from getting coolant deep inside? That could explain the temperature spikes at start-up because the sensor is on the opposite side of the block from the water pump. Early on the heat transfers through the block (instead of the capillaries with coolant) to the coolant system/thermostat where it then opens and starts to regulate the temperature. I did notice a fair amount of "sludge" at the drain cock when I first flushed it. I did a decent job of thoroughly flushing the system but obviously didn't get the hose into the block without removing the water pump.
 

kuboman

Member
Dec 6, 2009
725
4
16
Canada
It is physically impossible for you to get temp spikes a few seconds after starting a cold engine. That eliminates any mechanical problem. It has to be electrical involving the temp gauge.
 

BrianF8

New member

Equipment
ZD28F
Jun 4, 2014
21
0
0
Parkton, MD
The temp spikes occur probably within 30-60s of starting (they're not immediate). I'll test the temperature sensor separately tonight.